You're looking at the wrong file. Setting argv[0] doesn't update /proc/self/comm, it updates /proc/self/cmdline.

demo.c:

    #include <unistd.h>
    #include <string.h>
    
    int main(int argc, char *argv[]) {
    	strcpy(argv[0], "frob");
    	sleep(100);
    }

Terminal 1:

    $ make demo
    cc     demo.c   -o demo
    $ ./demo --greppable

Terminal 2:

    $ ps aux|grep greppable
    luke     25858  0.0  0.0   2164   752 pts/0    S+   00:32   0:00 frob o --greppable
    luke     25931  0.0  0.0   8192  2356 pts/5    S+   00:32   0:00 grep --color=auto greppable
    $ cat -v /proc/25858/cmdline
    frob^@o^@--greppable^@$